Laser-diode-excited laser apparatus, fiber laser apparatus, and fiber laser amplifier in which laser medium doped with one of Ho3+, SM3+, EU3+, DY3+, ER3+ and TB3+ is excited with GaN-based compound laser diode

ABSTRACT

A solid-state laser crystal constituting a laser-diode-excited solid-state laser apparatus or an optical fiber constituting a fiber laser apparatus or fiber laser amplifier is doped with one of Ho 3+ , Sm 3+ , Eu 3+ , Dy 3+ , Er 3+ , and Tb 3+  so that a laser beam is emitted from the solid-state laser crystal or the optical fiber, or incident light of the fiber laser amplifier is amplified, by one of the transitions from  5 S 2  to  5 I 7 , from  5 S 2  to  5 I 8 , from  4 G 5/2  to  6 H 5/2 , from  4 G 5/2  to  6 H 7/2 , from  4 F 3/2  to  6 H 11/2 , from  5 D 0  to  7 F 2 , from  4 F 9/2  to  6 H 13/2 , from  4 F 9/2  to  6 H 11/2 , from  4 S 3/2  to  4 I 15/2 , from  2 H 9/2  to  4 I 13/2 , and from  5 D 4  to  7 F 5 . The above solid-state laser crystal or optical fiber is excited with a GaN-based compound laser diode.

This is a divisional of Application No. 10/143,806 filed May 14, 2002now U.S. Pat. No. 6,816,532.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject matters disclosed in this specification are related to thesubject matters disclosed in the following copending, commonly-assignedU.S. patent applications:

(1) U.S. Pat. No. 6,125,132 filed by Yoji Okazaki (one of the applicantsof the present patent application) on Apr. 28, 1998 and entitled “LASERDIODE PUMPED SOLID STATE LASER, FIBER LASER AND FIBER AMPLIFIER,”corresponding to Japanese patent application Nos. 10(1998)-6369 and10(1998)-6370, which are laid open in Japanese Unexamined PatentPublication Nos. 11(1999)-17266 and 11(1999)-204862; and

(2) U.S. Ser. No. 09/621,241 filed by Yoji Okazaki (one of theapplicants of the present patent application) and Takayuki Katoh(another of the applicants of the present patent application) on Jul.21, 2000 and entitled “LASER-DIODE-PUMPED LASER APPARATUS IN WHICHPr-DOPED LASER MEDIUM IS PUMPED WITH GaN-BASED COMPOUND LASER DIODE,”corresponding to Japanese patent application Nos. 11(1999)-206817 and11(1999)-206573, which are laid open in Japanese Unexamined PatentPublication Nos. 2001-36168 and 2001-36175.

The contents of the above copending, commonly-assigned U.S. patentapplications (1) and (2) and the corresponding Japanese patentapplications are incorporated in this specification by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser-diode-excited solid-state laserapparatus in which a solid-state laser crystal doped with a rare-earthion is excited with a laser diode (semiconductor laser) so as to emit alaser beam.

The present invention also relates to a laser-diode-excited solid-statelaser apparatus in which a solid-state laser crystal doped with arare-earth ion is excited with a laser diode (semiconductor laser), andwhich is arranged to emit ultraviolet light.

The present invention further relates to a laser-diode-excited fiberlaser apparatus in which a core of an optical fiber doped with arare-earth ion is excited with a laser diode (semiconductor laser) so asto emit a laser beam.

The present invention furthermore relates to a laser-diode-excited fiberlaser amplifier in which a core of an optical fiber doped with arare-earth ion is excited with a laser diode (semiconductor laser) so asto amplify incident light by utilizing fluorescence generated by theexcitation of the core.

2. Description of the Related Art

(1) Solid-State Laser

Gas-laser-excited solid-state laser apparatuses in which a Pr³⁺-dopedsolid-state laser crystal is excited with a gas laser such as an Arlaser are known as disclosed in Journal of Applied Physics, vol. 48, No.2, pp. 650-653 (1977) and Applied Physics, B58, pp. 149-151 (1994).These solid-state laser apparatuses can generate a laser beam in a bluewavelength range of 470 to 490 nm by a transition from ³P₀ to ³H₄ and alaser beam in a green wavelength range of 520 to 550 nm by a transitionfrom ³P₁ to ³H₅. Therefore, the above solid-state laser apparatuses canbe used as light sources for recording a color image in a colorsensitive material.

In addition, another solid-state laser apparatus which emits a laserbeam having a wavelength in the blue or green wavelength range is known.For example, the Japanese Unexamined Patent Publication No.4(1992)-318988 corresponding to the Japanese Patent Application No.3(1991)-086405, which is assigned to the present assignee, discloses alaser-diode-excited solid-state laser apparatus in which a solid-statelaser beam is converted into a second harmonic, i.e., the wavelength ofthe solid-state laser beam is reduced, by arranging a nonlinear opticalcrystal in a resonator.

Further, InGaN-based compound laser diodes and ZnMgSSe-based compoundlaser diodes which emit laser beams in the blue and green wavelengthranges have recently been developed.

However, the light sources for use in recording a color image in a colorimage recording apparatus are required to be small in size, light inweight, and inexpensive. Nevertheless, the above gas-laser-excitedsolid-state laser apparatus using the Pr³⁺-doped solid-state lasercrystal is not suitable for use in recording a color image in a colorimage recording apparatus since the gas laser in the gas-laser-excitedsolid-state laser apparatus are large, heavy, and expensive.

On the other hand, since the efficiency of wavelength conversion in theconventional laser-diode-excited solid-state laser apparatuses in whicha wavelength of a solid-state laser beam is reduced by using a nonlinearoptical crystal is not sufficiently high, it is difficult to obtain highoutput power. In addition, in such laser-diode-excited solid-state laserapparatuses, an etalon or the like is inserted for limiting theoscillation mode to a single mode. Therefore, loss in the resonator isgreat, and thus achievement of high output power becomes more difficult.

Further, in order to match phases in the wavelength conversion in theabove laser-diode-excited solid-state laser apparatuses, highly accuratetemperature control is required, and therefore the outputs of thelaser-diode-excited solid-state laser apparatuses are not stable.Furthermore, the numbers of parts are increased by the provision of thenonlinear optical crystal and the etalon. Therefore, thelaser-diode-excited solid-state laser apparatuses become expensive.

When InGaN-based compound laser diodes are used, the oscillationwavelengths of the InGaN-based compound laser diodes increase withincrease in the indium contents, and theoretically it is possible toobtain laser beams in the blue wavelength range of 470 to 490 nm or inthe green wavelength range of 520 to 550 nm. However, since the qualityof the crystal deteriorates with the increase in the indium content, itis practically impossible to sufficiently increase the indium content,and the upper limit of the lengthened wavelength is about 450 nm.

In addition, blue light can be obtained by other laser diodes having anactive layer made of an InGaNAs or GaNAs material. The oscillationwavelengths in these laser diodes can also be increased by doping theactive layer with arsenic. However, since the quality of the crystalalso deteriorates with the increase in the arsenic content, the upperlimit of the wavelength realizing high output power is about 450 to 460nm.

Further, the conventional ZnMgSSe-based compound laser diodes cannotcontinuously oscillate at wavelengths below 500 nm at room temperature,and the lifetimes of the conventional ZnMgSSe-based compound laserdiodes are at most about a hundred hours.

In order to solve the above problems, the copending, commonly-assignedU.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent PublicationNo. 11(1999)-17266 disclose a laser-diode-excited solid-state laserapparatus which is inexpensive, and can emit a laser beam in the blue orgreen wavelength range with high efficiency, high output power, and highoutput stability. In this laser-diode-excited solid-state laserapparatus, a Pr³⁺-doped solid-state laser crystal is excited with aGaN-based compound laser diode.

(2) Ultraviolet Laser

Highly efficient, high output power ultraviolet lasers whichcontinuously oscillate in the ultraviolet wavelength range are required,for example, for applications in ultraviolet lithography, fluorometricanalysis of organic cells using laser excitation, and the like.

GaN-based compound semiconductor lasers having an active layer made ofan InGaN, InGaNAs, or GaNAs material are known as lasers which oscillatein the ultraviolet wavelength range. Recently, GaN-based compoundsemiconductor lasers which can continuously oscillate for a thousandhours at the wavelength of 400 nm with output power of severalmilliwatts have been provided.

On the other hand, wavelength-conversion solid-state lasers which outputultraviolet laser beams having wavelengths of 400 nm or below are known.In these wavelength-conversion solid-state lasers, wavelengths of laseroscillation light are shortened to the ultraviolet wavelengths by secondharmonic generation (SHG) or third harmonic generation (THG) usingnonlinear optical crystals.

However, the conventional GaN-based compound semiconductor lasers cannotemit laser light with output power of 100 mW or more in a singletransverse mode, although such laser light is required in manyapplications. In addition, the oscillation efficiency in theconventional GaN-based compound semiconductor lasers which emit laserlight having wavelengths of 380 nm or below is low, and the lifetimes ofsuch GaN-based compound semiconductor lasers are very short.

On the other hand, wavelength-conversion solid-state lasers which outputultraviolet laser beams having wavelengths of 400 nm or below are known.In these wavelength-conversion solid-state lasers, wavelengths of laseroscillation light are shortened to the ultraviolet wavelengths by secondharmonic generation (SHG) or third harmonic generation (THG) usingnonlinear optical crystals.

However, solid-state laser mediums which realize efficient oscillationin the wavelength range of 700 to 800 nm have not yet been found.Therefore, it is difficult to obtain ultraviolet laser beams with highoutput power from the wavelength-conversion solid-state lasers in whichthe wavelengths of the laser light are shortened by second harmonicgeneration (SHG).

In addition, the efficiency of the wavelength-conversion solid-statelasers in which the wavelengths of the laser light are shortened bythird harmonic generation (THG) is essentially low, and the conventionalTHG wavelength-conversion solid-state lasers can oscillate in only apulse mode. In order to realize continuous oscillation, i.e., in orderto maintain resonance of THG light of the fundamental wave, highlyaccurate temperature adjustment of a resonator with a precision of 0.01°C. is required. However, such accurate temperature adjustment ispractically difficult in terms of cost.

In order to solve the above problems, the copending, commonly-assignedU.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent PublicationNo. 2001-36175 disclose a laser-diode-excited solid-state laserapparatus in which a solid-state laser beam is converted into a secondharmonic by using an optical wavelength conversion element so thatultraviolet light is obtained.

The above laser-diode-excited solid-state laser apparatus comprises: asolid-state laser crystal which is doped with at least one rare-earthion including at least Pr³⁺; a laser diode which has an active layermade of one of InGaN, InGaNAs, and GaNAs materials, and emits anexcitation laser beam for exciting the solid-state laser crystal; and anoptical wavelength conversion element which performs wavelengthconversion on a solid-state laser beam generated by excitation of thesolid-state laser crystal so as to generate ultraviolet laser light.

Although the laser-diode-excited solid-state laser apparatus disclosedin the U.S. Ser. No. 09/621,241 and the Japanese Unexamined PatentPublication No. 2001-36175 can solve the aforementioned problems, thewavelength of the ultraviolet light which can be generated by thedisclosed laser-diode-excited solid-state laser apparatus is limited toabout 360 nm.

(3) Fiber Laser

As disclosed in the Technical Report of the Institute of Electronics,information and Communication Engineers in Japan, LQE95-30 (1995) p. 30and Optics Communications 86 (1991) p. 337, laser-diode-excited fiberlaser apparatuses which comprise a laser diode and an optical fiberhaving a core made of a Pr³+-doped fluoride are known. In thelaser-diode-excited fiber laser apparatuses, the optical fiber isexcited with the laser diode so as to generate a laser beam.

In addition, as disclosed in the above references, optical fiberamplifiers which comprise a laser diode and an optical fiber having aPr³⁺-doped core are also known. In these optical fiber amplifiers, theoptical fiber is excited with the laser diode so that fluorescent lightis generated by the excitation of the optical fiber, and incident lightof the optical fiber is amplified by the energy of the fluorescent lightwhen the wavelength of the incident light is included in the wavelengthrange of the fluorescent light.

Further, Optics Communications 86 (1991) p. 337 discloses anAr-laser-excited, Pr³⁺-doped fiber laser apparatuses, and laseroscillations at the wavelengths of 491, 520, 605, and 635 nm usingexcitation light having a wavelength of 476.5 nm have been reported.

The above laser-diode-excited fiber laser apparatuses andAr-laser-excited, Pr³⁺-doped fiber laser apparatuses can emit blue orgreen laser beams, and the above optical fiber amplifiers can amplifyblue or green laser beams. In this respect, it is considered that theseapparatuses and amplifiers can be used as constituents of light sourcesfor recording a color image in a color sensitive material.

However, in order to operate the above Ar-laser-excited, Pr³⁺-dopedfiber laser apparatuses or Pr³⁺-doped fiber laser amplifiers with highpower of a few watts to several tens of watts, for example, forrecording a color image, a water cooling system is necessary. Therefore,the size is increased, and the lifetime and the efficiency are reduced.

In order to solve the above problems, the copending, commonly-assignedU.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent PublicationNo. 11(1999)-204862 disclose a fiber laser apparatus which canefficiently emit a laser beam in a blue or green wavelength range withhigh output power and high stability in the output and beam quality, andcan be formed in a small size. In this fiber laser apparatus, an opticalfiber having a core doped with Pr³⁺ is excited with a GaN-based compoundlaser diode.

In addition, the U.S. Pat. No. 6,125,132 and the Japanese UnexaminedPatent Publication No. 11(1999)-204862 also disclose a fiber laseramplifier which can efficiently amplify a laser beam in a blue or greenwavelength range with high output power and high stability in the outputand beam quality, and can be formed in a small size. In this fiber laseramplifier, an optical fiber having a core doped with Pr³⁺is excited witha GaN-based compound laser diode so as to amplify incident light of theoptical fiber when the wavelength of the incident light is in thewavelength range of fluorescent light generated by the excitation of theoptical fiber.

Further, the U.S. Ser. No. 09/621,241 and the Japanese Unexamined PatentPublication No. 2001-36168 disclose a fiber laser apparatus in which anoptical fiber having a core codoped with Pr³⁺ and at least one of Er³⁺,Ho³⁺, Dy³⁺, Eu³⁺, Sm³⁺, Pm³⁺, and Nd³⁺ is excited with a GaN-basedcompound laser diode. The U.S. Ser. No. 09/621,241 and the JapaneseUnexamined Patent Publication No. 2001-36168 also disclose a fiber laseramplifier in which an optical fiber having a core codoped with Pr³⁺ andat least one of Er³⁺, Ho³⁺, Dy³⁺, Eu³⁺, Sm³⁺, Pm³⁺, and Nd³⁺ is excitedwith a GaN-based compound laser diode so as to amplify incident light ofthe optical fiber when the wavelength of the incident light is in thewavelength range of fluorescent light generated by the excitation of theoptical fiber.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide alaser-diode-excited solid-state laser apparatus which uses a GaN-basedcompound laser diode as an excitation light source, and can emit laserlight in a wide wavelength range which is not covered by theconventional laser-diode-excited solid-state laser apparatuses whichuses a Pr³⁺-doped solid-state laser crystal.

A second object of the present invention is to provide alaser-diode-excited solid-state laser apparatus which is inexpensive,and can continuously emit ultraviolet light having a wavelength longeror shorter than 360 nm with high output power and high efficiency.

A third object of the present invention is to provide a fiber laserapparatus which uses a GaN-based compound laser diode as an excitationlight source, and can emit laser light in a wide wavelength range whichis not covered by the conventional fiber laser apparatuses which use aGaN-based compound laser diode as an excitation light source.

A fourth object of the present invention is to provide a fiber laseramplifier which uses a GaN-based compound laser diode as an excitationlight source, and can amplify laser light in a wide wavelength rangewhich is not covered by the conventional fiber laser amplifiers whichuse a GaN-based compound laser diode as an excitation light source.

(I) According to the first aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Ho³⁺, and emits asolid-state laser beam generated by one of a first transition from ⁵S₂to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ when the solid-statelaser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the first aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁵S₂ to ⁵I₇ and is in the wavelength range of 740        to 760 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁵S₂ to ⁵I₈ and is in the wavelength range of 540        to 560 nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Ho³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Ho³⁺ is 420 nm.

(II) According to the second aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Sm³⁺, and emits asolid-state laser beam generated by one of a first transition from⁴G_(5/2) to ⁶H_(5/2), a second transition from ⁴G_(5/2) to ⁶H_(7/2), anda third transition from ⁴F_(3/2) to ⁶H_(11/2) when the solid-state lasercrystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the second aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iv).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength        range of 556 to 576 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength        range of 605 to 625 nm.    -   (iii) The solid-state laser beam is generated by the third        transition from ⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength        range of 640 to 660 nm.    -   (iv) The solid-state laser crystal is doped with no rare-earth        ion other than Sm³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Sm³⁺ is 404 nm.

(III) According to the third aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Eu³⁺, and emits asolid-state laser beam by a transition from ⁵D₀ to ⁷F₂ when thesolid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the third aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

-   -   (i) The solid-state laser beam is in the wavelength range of 579        to 599 nm.    -   (ii) The solid-state laser crystal is doped with no rare-earth        ion other than Eu³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Eu³⁺ is 394 nm.

(IV) According to the fourth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Dy³⁺, and emits asolid-state laser beam generated by one of a first transition from⁴F_(9/2) to ⁶H_(13/2) and a second transition from ⁴F_(9/2) to ⁶H_(11/2)when the solid-state laser crystal is excited with the excitation laserbeam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the fourth aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴F_(9/2) to ⁶H_(13/2) and is in the wavelength        range of 562 to 582 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength        range of 654 to 674 nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Dy³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Dy³⁺ is 390 nm.

(V) According to the fifth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Er³⁺, and emits asolid-state laser beam generated by one of a first transition from⁴S_(3/2) to ⁴I_(15/2) and a second transition from ²H_(9/2) to ⁴I_(13/2)when the solid-state laser crystal is excited with the excitation laserbeam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the fifth aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength        range of 530 to 550 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ²H_(9/2) to ⁴I_(13/2) and is in the wavelength        range of 544 to 564 nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Er³⁺

The excitation wavelength of the solid-state laser crystal which isdoped with Er³⁺ is 406 or 380 nm.

(VI) According to the sixth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Tb³⁺, and emits asolid-state laser beam generated by a transition from ⁵D₄ to ⁷F₅ whenthe solid-state laser crystal is excited with the excitation laser beam.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the sixth aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

-   -   (i) The solid-state laser beam is in the wavelength range of 530        to 550 nm.    -   (ii) The solid-state laser crystal is doped with no rare-earth        ion other than Tb³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Tb³⁺ is 380 nm.

In addition, in the first to sixth aspects of the present invention, theGaN-based compound laser diode may have an active layer made of one ofInGaN, InGaNAs, and GaNAs materials.

(VII) The advantages of the first to sixth aspects of the presentinvention are as follows.

-   -   (i) Since the rare-earth ions, Ho³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, and        Tb³⁺ have their absorption bands in the wavelength range of 380        to 420 nm, it is relatively easy to excite the rare-earth ions        with a GaN-based compound laser diode. The wavelength range of        380 to 430 nm is a wavelength range in which the GaN-based        compound laser diodes can oscillate with relative ease. In        particular, the currently available GaN-based compound laser        diodes can achieve their maximum output power in the wavelength        range of 400 to 410 nm. Therefore, when a solid-state laser        crystal doped with at least one of the rare-earth ions, Ho³⁺,        Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, and Tb³⁺ is excited with a GaN-based        compound laser diode, it is possible to make a great portion of        the excitation light absorbed by the solid-state laser crystal,        and achieve high efficiency and high output power.    -   (ii) In addition, as individually exemplified before; the        wavelength bands of the fluorescence generated by the excitation        of the solid-state laser crystals doped with the rare-earth        ions, Ho³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺, and Tb³⁺ are distributed in        a wide wavelength range. Therefore, it is possible to realize a        laser-diode-excited solid-state laser apparatus which can emit        laser light having a wavelength which no laser light capable of        being generated by the conventional laser-diode-excited        solid-state laser apparatuses has.    -   (iii) On the other hand, the thermal conductivity coefficients        of the GaN-based compound laser diodes are about 130 W/m° C.,        and much greater than the thermal conductivity coefficients of        the ZnMgSSe-based compound laser diodes, which are about 4        W/m° C. In addition, since the dislocation mobility in the        GaN-based compound laser diodes is very low, compared with that        in the ZnMgSSe-based compound laser diodes, the COD        (catastrophic optical damage) thresholds of the GaN-based        compound laser diodes are very high. Therefore, it is easy to        obtain GaN-based compound laser diodes having a long lifetime        and high output power. Since the laser-diode-excited solid-state        laser apparatuses according to the first to sixth aspects of the        present invention use a GaN-based compound laser diode as an        excitation light source, the laser-diode-excited solid-state        laser apparatuses according to the first to sixth aspects of the        present invention can have a long lifetime, and emit a laser        beam with high output power.

(VIII) According to the seventh aspect of the present invention, thereis provided a laser-diode-excited solid-state laser apparatus including:a laser diode which has an active layer made of one of InGaN, InGaNAs,and GaNAs materials, and emits an excitation laser beam; a solid-statelaser crystal which is doped with at least one rare-earth ion includingHo³⁺, and emits a solid-state laser beam generated by one of a firsttransition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈ whenthe solid-state laser crystal is excited with the excitation laser beam;and an optical wavelength conversion element which converts thesolid-state laser beam into ultraviolet laser light by wavelengthconversion.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the seventh aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁵S₂ to ⁵I₇ and has a wavelength of about 750 nm,        and the ultraviolet laser light has a wavelength of about 375        nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁵S₂ to ⁵I₈ and has a wavelength of about 550 nm,        and the ultraviolet laser light has a wavelength of about 275        nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Ho³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Ho³⁺ is 420 nm.

(IX) According to the eighth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Sm³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴G_(5/2) to ⁶H_(5/2), a second transition from ⁴G_(5/2)to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to ⁶H_(11/2) when thesolid-state laser crystal is excited with the excitation laser beam; andan optical wavelength conversion element which converts the solid-statelaser beam into ultraviolet laser light by wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the eighth aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iv).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴G_(5/2) to ⁶H_(5/2) and has a wavelength of        about 566 nm, and the ultraviolet laser light has a wavelength        of about 283 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁴G_(5/2) to ⁶H_(7/2) and has a wavelength of        about 615 nm, and the ultraviolet laser light has a wavelength        of about 308 nm.    -   (iii) The solid-state laser beam is generated by the third        transition from ⁴F_(3/2) to ⁶H_(11/2) and has a wavelength of        about 650 nm, and the ultraviolet laser light has a wavelength        of about 325 nm.    -   (iv) The solid-state laser crystal is doped with no rare-earth        ion other than Sm³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Sm³⁺ is 404 nm.

(X) According to the ninth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Eu³⁺,and emits a solid-state laser beam generated by a transition from ⁵D₀ to⁷F₂ when the solid-state laser crystal is excited with the excitationlaser beam; and an optical wavelength conversion element which convertsthe solid-state laser beam into ultraviolet laser light by wavelengthconversion.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the ninth aspect of the present invention may also have oneor any possible combination of the following additional features (i) and(ii).

-   -   (i) The solid-state laser beam has a wavelength of about 589 nm,        and the ultraviolet laser light has a wavelength of about 295        nm.    -   (ii) The solid-state laser crystal is doped with no rare-earth        ion other than Eu⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Eu³⁺ is 394 nm.

(XI) According to the tenth aspect of the present invention, there isprovided a laser-diode-excited solid-state laser apparatus including: alaser diode which has an active layer made of one of InGaN, InGaNAs, andGaNAs materials, and emits an excitation laser beam; a solid-state lasercrystal which is doped with at least one rare-earth ion including Dy³⁺,and emits a solid-state laser beam generated by one of a firsttransition from ⁴F_(9/2) to ⁶H_(13/2) and a second transition from⁴F_(9/2) to ⁶H_(11/2) when the solid-state laser crystal is excited withthe excitation laser beam; and an optical wavelength conversion elementwhich converts the solid-state laser beam into ultraviolet laser lightby wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the tenth aspect of the present invention may also have oneor any possible combination of the following additional features (i) to(iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴F_(9/2) to ⁶H_(13/2) and has a wavelength of        about 572 nm, and the ultraviolet laser light has a wavelength        of about 286 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ⁴F_(9/2) to ⁶H_(11/2) and has a wavelength of        about 664 nm, and the ultraviolet laser light has a wavelength        of about 332 nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Dy³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Dy³⁺ is 390 nm.

(XII) According to the eleventh aspect of the present invention, thereis provided a laser-diode-excited solid-state laser apparatus including:a laser diode which has an active layer made of one of InGaN, InGaNAs,and GaNAs materials, and emits an excitation laser beam; a solid-statelaser crystal which is doped with at least one rare-earth ion includingEr³⁺, and emits a solid-state laser beam generated by one of a firsttransition from ⁴S_(3/2) to ⁴I_(15/2) and a second transition from²H_(9/2) to ⁴I_(13/2) when the solid-state laser crystal is excited withthe excitation laser beam; and an optical wavelength conversion elementwhich converts the solid-state laser beam into ultraviolet laser lightby wavelength conversion.

Preferably, the laser-diode-excited solid-state laser apparatusaccording to the eleventh aspect of the present invention may also haveone or any possible combination of the following additional features (i)to (iii).

-   -   (i) The solid-state laser beam is generated by the first        transition from ⁴S_(3/2) to ⁴I_(15/2) and has a wavelength of        about 540 nm, and the ultraviolet laser light has a wavelength        of about 270 nm.    -   (ii) The solid-state laser beam is generated by the second        transition from ²H_(9/2) to ⁴I_(13/2) and has a wavelength of        about 554 nm, and the ultraviolet laser light has a wavelength        of about 277 nm.    -   (iii) The solid-state laser crystal is doped with no rare-earth        ion other than Er³⁺.

The excitation wavelength of the solid-state laser crystal which isdoped with Er³⁺ is 406 or 380 nm.

In addition, in the seventh to eleventh aspects of the presentinvention, the optical wavelength conversion element may be realized bya nonlinear optical crystal having a periodic domain-inverted structure.

(XIII) The advantages of the seventh to eleventh aspects of the presentinvention are as follows.

-   -   (i) When a Ho³⁺-doped solid-state laser crystal, e.g., a        Ho³⁺-doped YAG crystal, is excited with a GaN-based compound        laser diode (at an excitation wavelength of 420 nm), a        solid-state laser beam in the near infrared range is generated        by a transition from ⁵S₂ to ⁵I₇. Therefore, when this        solid-state laser beam is converted into a second harmonic by        wavelength conversion using an optical wavelength conversion        element, ultraviolet light having a high intensity and a        wavelength longer than 360 nm can be obtained. That is, a        solid-state laser beam having a wavelength of, for example,        about 750 nm can be obtained by the above transition. Thus, when        this solid-state laser beam is converted into a second harmonic,        ultraviolet light having a high intensity and a wavelength of        about 375 nm is obtained.    -   (ii) In addition, when the Ho³⁺-doped solid-state laser crystal        is excited with the GaN-based compound laser diode, a        solid-state laser beam having a wavelength of about 550 nm is        generated by a transition from ⁵S₂ to ⁵I₈. Therefore, when this        solid-state laser beam is converted into a second harmonic by        wavelength conversion using an optical wavelength conversion        element, ultraviolet light having a high intensity and a        wavelength of about 275 nm, which is shorter than 360 nm, is        obtained.    -   (iii) Although the construction for realizing the wavelength        conversion of the solid-state laser beam into a third harmonic        is complex, the construction for realizing the wavelength        conversion of the solid-state laser beam into a second harmonic        is simple. Therefore, the laser-diode-excited solid-state laser        apparatus using the wavelength conversion into a second harmonic        is inexpensive.    -   (iv) In the laser-diode-excited solid-state laser apparatus        according to the eighth aspect of the present invention, in        which a Sm³⁺-doped solid-state laser crystal is used,        solid-state laser beams having wavelengths of about 566, 615,        and 650 nm can be generated by excitation, for example, at the        excitation wavelength of 404 nm, as explained before. Therefore,        when these solid-state laser beams are converted into second        harmonics by wavelength conversion, ultraviolet light beams        having wavelengths of about 283, 308, and 325 nm can be        obtained, respectively.    -   (v) In the laser-diode-excited solid-state laser apparatus        according to the ninth aspect of the present invention, in which        a Eu³⁺-doped solid-state laser crystal is used, a solid-state        laser beam having a wavelength of about 589 nm can be generated        by excitation, for example, at the excitation wavelength of 394        nm, as explained before. Therefore, when this solid-state laser        beam is converted into a second harmonic by wavelength        conversion, an ultraviolet light beam having a wavelength of        about 295 nm can be obtained.    -   (vi) In the laser-diode-excited solid-state laser apparatus        according to the tenth aspect of the present invention, in which        a Dy³⁺-doped solid-state laser crystal is used, solid-state        laser beams having wavelengths of about 572 and 664 nm can be        generated by excitation, for example, at the excitation        wavelength of 390 nm, as explained before. Therefore, when these        solid-state laser beams are converted into second harmonics by        wavelength conversion, ultraviolet light beams having        wavelengths of about 286 and 332 nm can be obtained,        respectively.    -   (vii) In the laser-diode-excited solid-state laser apparatus        according to the eleventh aspect of the present invention, in        which an Er³⁺-doped solid-state laser crystal is used,        solid-state laser beams having wavelengths of about 540 and 554        nm can be generated by excitation, for example, at the        excitation wavelength of 406 or 380 nm, as explained before.        Therefore, when these solid-state laser beams are converted into        second harmonics by wavelength conversion, ultraviolet light        beams having wavelengths of about 270 and 277 nm can be        obtained, respectively.    -   (viii) As explained before, the rare-earth ions, Ho³⁺, Sm³⁺,        Eu³⁺, Dy³⁺, and Er³⁺ have their absorption bands in the        wavelength range of 380 to 420 nm, in which the currently        available GaN-based compound laser diodes can easily oscillate.        In particular, the currently available GaN-based compound laser        diodes can achieve their maximum output power in the wavelength        range of 400 to 410 nm. Since the solid-state laser crystals        respectively doped with the rare-earth ions, Ho³⁺, Sm³⁺, Eu³⁺,        Dy³⁺, and Er³⁺ are excited with a GaN-based compound laser diode        in the laser-diode-excited solid-state laser apparatuses        according to the seventh to eleventh aspects of the present        invention, a great portion of the excitation light is absorbed        by the solid-state laser crystal, and high efficiency and high        output power can be achieved.    -   (ix) In addition, as explained before, the GaN-based compound        laser diodes have a great thermal conductivity coefficient and a        high COD (catastrophic optical damage) threshold. Therefore, it        is easy to obtain GaN-based compound laser diodes having a long        lifetime and high output power. The laser-diode-excited        solid-state laser apparatuses according to the seventh to        eleventh aspects of the present invention, in which a GaN-based        compound laser diode is used as an excitation light source, can        have a long lifetime, and emit a laser beam with high output        power.

(XIV) According to the twelfth aspect of the present invention, there isprovided a fiber laser apparatus including: a GaN-based compound laserdiode which emits a first laser beam; and an optical fiber which has acore doped with Ho³⁺ and emits a second laser beam generated by one of afirst transition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂ to ⁵I₈when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the twelfth aspect ofthe present invention may also have one or any possible combination ofthe following additional features (i) to (iii).

-   -   (i) The second laser beam is generated by the first transition        from ⁵S₂ to ⁵I₇ and is in the wavelength range of 740 to 760 nm.    -   (ii) The second laser beam is generated by the second transition        from ⁵S₂ to ⁵I₈ and is in the wavelength range of 540 to 560 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Ho³⁺.

The excitation wavelength of the core of the optical fiber doped withHo³⁺ is 420 nm.

(XV) According to the thirteenth aspect of the present invention, thereis provided a fiber laser apparatus including: a GaN-based compoundlaser diode which emits a first laser beam; and an optical fiber whichhas a core doped with Sm³⁺, and emits a second laser beam generated byone of a first transition from ⁴G_(5/2) to ⁶H_(5/2), a second transitionfrom ⁴G_(5/2) to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to⁶H_(11/2) when the optical fiber is excited with the first laser beam.

Preferably, the fiber laser apparatus according to the thirteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) to (iv).

-   -   (i) The second laser beam is generated by the first transition        from ⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength range of 556        to 576 nm.    -   (ii) The second laser beam is generated by the second transition        from ⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength range of 605        to 625 nm.    -   (iii) The second laser beam is generated by the third transition        from ⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength range of 640        to 660 nm.    -   (iv) The core of the optical fiber is doped with no rare-earth        ion other than Sm³⁺.

The excitation wavelength of the core of the optical fiber doped withSm³⁺ is 404 nm.

(XVI) According to the fourteenth aspect of the present invention, thereis provided a fiber laser apparatus including: a GaN-based compoundlaser diode which emits a first laser beam; and an optical fiber whichhas a core doped with Eu³⁺, and emits a second laser beam generated by atransition from ⁵D₀ to ⁷F₂ when the optical fiber is excited with thefirst laser beam.

Preferably, the fiber laser apparatus according to the fourteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) and (ii).

-   -   (i) The second laser beam is in the wavelength range of 579 to        599 nm.    -   (ii) The core of the optical fiber is doped with no rare-earth        ion other than Eu³⁺.

The excitation wavelength of the core of the optical fiber doped withEu³⁺ is 394 nm.

(XVII) According to the fifteenth aspect of the present invention, thereis provided a fiber laser apparatus including: a GaN-based compoundlaser diode which emits a first laser beam; and an optical fiber whichhas a core doped with Dy³⁺, and emits a second laser beam generated byone of a first transition from ⁴F_(9/2) to ⁶H_(13/2) and a secondtransition from ⁴F_(9/2) to ⁶H_(11/2) when the optical fiber is excitedwith the first laser beam.

Preferably, the fiber laser apparatus according to the fifteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) to (iii).

-   -   (i) The second laser beam is generated by the first transition        from ⁴F_(9/2) to ⁶H_(13/2) and is in the wavelength range of 562        to 582 nm.    -   (ii) The second laser beam is generated by the second transition        from ⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength range of 654        to 674 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Dy³⁺.

The excitation wavelength of the core of the optical fiber doped withDy³⁺ is 390 nm.

(XVIII) According to the sixteenth aspect of the present invention,there is provided a fiber laser apparatus including: a GaN-basedcompound laser diode which emits a first laser beam; and an opticalfiber which has a core doped with Er³⁺, and emits a second laser beamgenerated by one of a first transition from ⁴S_(3/2) to ⁴I_(15/2) and asecond transition from ²H_(9/2) to ⁴I_(13/2) when the optical fiber isexcited with the first laser beam.

Preferably, the fiber laser apparatus according to the sixteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) to (iii).

-   -   (i) The second laser beam is generated by the first transition        from ⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength range of 530        to 550 nm.    -   (ii) The second laser beam is generated by the second transition        from ²H_(9/2) to ⁴I_(13/2) and is in the wavelength range of 544        to 564 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Er³⁺.

The excitation wavelength of the core of the optical fiber doped withEr³⁺ is 406 or 380 nm.

(XIX) According to the seventeenth aspect of the present invention,there is provided a fiber laser apparatus including: a GaN-basedcompound laser diode which emits a first laser beam; and an opticalfiber which has a core doped with Tb³⁺, and emits a second laser beamgenerated by a transition from ⁵D₄ to ⁷F₅ when the optical fiber isexcited with the first laser beam.

Preferably, the fiber laser apparatus according to the seventeenthaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

-   -   (i) The second laser beam is in the wavelength range of 530 to        550 nm.    -   (ii) The core of the optical fiber is doped with no rare-earth        ion other than Tb³⁺.        The excitation wavelength of the core of the optical fiber doped        with Tb³⁺ is 380 nm.

(XX) According to the eighteenth aspect of the present invention, thereis provided a fiber laser amplifier including: a GaN-based compoundlaser diode which emits an excitation laser beam; and an optical fiberwhich has a core doped with Ho³⁺, and amplifies incident light which hasa wavelength within a wavelength range of fluorescence generated by oneof a first transition from ⁵S₂ to ⁵I₇ and a second transition from ⁵S₂to ⁵I₈ when the optical fiber is excited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the eighteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) to (iii).

-   -   (i) The fluorescence is generated by the first transition from        ⁵S₂ to ⁵I₇ and is in the wavelength range of 740 to 760 nm.    -   (ii) The fluorescence is generated by the second transition from        ⁵S₂ to ⁵I₈ and is in the wavelength range of 540 to 560 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Ho³⁺.

The excitation wavelength of the optical fiber which is doped with Ho³⁺is 420 nm.

(XXI) According to the nineteenth aspect of the present invention, thereis provided a fiber laser amplifier including: a GaN-based compoundlaser diode which emits an excitation laser beam; and an optical fiberwhich has a core doped with Sm³⁺, and amplifies incident light which hasa wavelength within a wavelength range of fluorescence generated by oneof a first transition from ⁴G_(5/2) to ⁶H_(5/2), a second transitionfrom ⁴G_(5/2) to ⁶H_(7/2), and a third transition from ⁴F_(3/2) to⁶H_(11/2) when the optical fiber is excited with the excitation laserbeam.

Preferably, the fiber laser amplifier according to the nineteenth aspectof the present invention may also have one or any possible combinationof the following additional features (i) to (iv).

-   -   (i) The fluorescence is generated by the first transition from        ⁴G_(5/2) to ⁶H_(5/2) and is in the wavelength range of 556 to        576 nm.    -   (ii) The fluorescence is generated by the second transition from        ⁴G_(5/2) to ⁶H_(7/2) and is in the wavelength range of 605 to        625 nm.    -   (iii) The fluorescence is generated by the third transition from        ⁴F_(3/2) to ⁶H_(11/2) and is in the wavelength range of 640 to        660 nm.    -   (iv) The core of the optical fiber is doped with no rare-earth        ion other than Sm³⁺.

The excitation wavelength of the core of the optical fiber doped withSm³⁺ is 404 nm.

(XXII) According to the twentieth aspect of the present invention, thereis provided a fiber laser amplifier including: a GaN-based compoundlaser diode which emits an excitation laser beam; and an optical fiberwhich has a core doped with Eu³⁺, and amplifies incident light which hasa wavelength within a wavelength range of fluorescence generated by atransition from ⁵D₀ to ⁷F₂ when the optical fiber is excited with theexcitation laser beam.

Preferably, the fiber laser amplifier according to the twentieth aspectof the present invention may also have one or any possible combinationof the following additional features (i) and (ii).

-   -   (i) The fluorescence is in the wavelength range of 579 to 599        nm.    -   (ii) The core of the optical fiber is doped with no rare-earth        ion other than Eu³⁺.

The excitation wavelength of the core of the optical fiber doped withEu³⁺is 394 nm.

(XXIII) According to the twenty-first aspect of the present invention,there is provided a fiber laser amplifier including: a GaN-basedcompound laser diode which emits an excitation laser beam; and anoptical fiber which has a core doped with Dy³⁺, and amplifies incidentlight which has a wavelength within a wavelength range of fluorescencegenerated by one of a first transition from ⁴F_(9/2) to ⁶H_(13/2) and asecond transition from ⁴F_(9/2) to ⁶H_(11/2) when the optical fiber isexcited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the twenty-firstaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

-   -   (i) The fluorescence is generated by the first transition from        ⁴F_(9/2) to ⁶H_(13/2) and is in the wavelength range of 562 to        582 nm.    -   (ii) The fluorescence is generated by the second transition from        ⁴F_(9/2) to ⁶H_(11/2) and is in the wavelength range of 654 to        674 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Dy³⁺.

The excitation wavelength of the optical fiber which is doped with Dy³⁺is 390 nm.

(XXIV) According to the twenty-second aspect of the present invention,there is provided a fiber laser amplifier including: a GaN-basedcompound laser diode which emits an excitation laser beam; and anoptical fiber which has a core doped with Er³⁺, and amplifies incidentlight which has a wavelength within a wavelength range of fluorescencegenerated by one of a first transition from ⁴S_(3/2) to ⁴I_(15/2) and asecond transition from ²H_(9/2) to ⁴I_(13/2) when the optical fiber isexcited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the twenty-secondaspect of the present invention may also have one or any possiblecombination of the following additional features (i) to (iii).

-   -   (i) The fluorescence is generated by the first transition from        ⁴S_(3/2) to ⁴I_(15/2) and is in the wavelength range of 530 to        550 nm.    -   (ii) The fluorescence is generated by the second transition from        ²H_(9/2) to ⁴I_(13/2) and is in the wavelength range of 544 to        564 nm.    -   (iii) The core of the optical fiber is doped with no rare-earth        ion other than Er³⁺.

The excitation wavelength of the core of the optical fiber doped withEr³⁺ is 406 or 380 nm.

(XXV) According to the twenty-third aspect of the present invention,there is provided a fiber laser amplifier including: a GaN-basedcompound laser diode which emits an excitation laser beam; and anoptical fiber which has a core doped with Tb³⁺, and amplifies incidentlight which has a wavelength within a wavelength range of fluorescencegenerated by a transition from ⁵D₄ to ⁷F₅ when the optical fiber isexcited with the excitation laser beam.

Preferably, the fiber laser amplifier according to the twenty-thirdaspect of the present invention may also have one or any possiblecombination of the following additional features (i) and (ii).

-   -   (i) The fluorescence is in the wavelength range of 530 to 550        nm.    -   (ii) The core of the optical fiber is doped with no rare-earth        ion other than Tb³⁺.

The excitation wavelength of the core of the optical fiber doped withTb³⁺ is 380 nm.

In addition, in the twelfth to twenty-third aspects of the presentinvention, the GaN-based compound laser diode may have an active layermade of one of InGaN, InGaNAs, and GaNAs materials.

(XXVI) The advantages of the twelfth to twenty-third aspects of thepresent invention are as follows.

-   -   (i) For similar reasons to those explained in paragraph (VII)        (i), in the fiber laser apparatuses and the fiber laser        amplifiers according to the twelfth to seventeenth aspects of        the present invention, a great portion of the excitation light        is absorbed by the optical fiber, and high efficiency and high        output power can be achieved.    -   (ii) For similar reasons to those explained in paragraph (VII)        (ii), the fiber laser apparatuses according to the twelfth to        seventeenth aspects of the present invention can emit laser        light having a wavelength which no laser light capable of being        generated by the conventional fiber laser apparatuses has, and        the fiber laser amplifiers according to the eighteenth to        twenty-third aspects of the present invention can amplify laser        light having a wavelength which no laser light capable of being        amplified by the conventional fiber laser amplifiers has.    -   (iii) For the same reasons as those explained in paragraph (VII)        (iii), the GaN-based compound laser diodes have a thermal        conductivity coefficient of about 130 W/m° C., which is much        greater than the thermal conductivity coefficient of the        ZnMgSSe-based compound laser diodes, which is about 4 W/m° C. In        addition, since the dislocation mobility in the GaN-based        compound laser diodes is very low, compared with that in the        ZnMgSSe-based compound laser diodes, the COD (catastrophic        optical damage) thresholds of the GaN-based compound laser        diodes are very high. Therefore, it is easy to obtain GaN-based        compound laser diodes having a long lifetime and high output        power. Since the fiber laser apparatuses and the fiber laser        amplifiers according to the twelfth to twenty-third aspects of        the present invention use a GaN-based compound laser diode as an        excitation light source, the laser-diode-excited solid-state        laser apparatuses have a long lifetime, and can emit or amplify        a laser beam with high output power.

(XXVII) In the constructions according to the first to twenty-thirdaspects of the present invention, the GaN-based compound laser diodesused as an excitation light source may be a single-longitudinal-mode,single-transverse-mode, broad-area, phased-array, or MOPA (masteroscillator power amplifier) type high power laser diode. In addition,one or more GaN-based compound laser diodes may be used in theconstructions according to the first to twenty-third aspects of thepresent invention. In this case, the constructions according to thefirst to seventeenth aspects of the present invention can emit a laserbeam with further higher output power, e.g., on the order of 1 W.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating an outline of the construction of alaser-diode-excited solid-state laser apparatus as a first embodiment ofthe present invention.

FIG. 2 is a side view illustrating an outline of the construction of alaser-diode-excited solid-state laser apparatus as a seventh embodimentof the present invention.

FIG. 3 is a side view illustrating an outline of the construction of afiber laser apparatus as a twelfth embodiment of the present invention.

FIG. 4 is a cross-sectional view of an optical fiber used in the fiberlaser apparatus of FIG. 3.

FIG. 5 is a side view illustrating an outline of the construction of afiber laser amplifier as an eighteenth embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are explained in detail below withreference to drawings.

First Embodiment

The first embodiment of the present invention is explained below. FIG. 1is a side view illustrating an outline of the construction of alaser-diode-excited solid-state laser apparatus as the first embodimentof the present invention.

The laser-diode-excited solid-state laser apparatus of FIG. 1 comprisesa laser diode 11, a condenser lens 12, and a Ho³⁺-doped solid-statecrystal 13. The laser diode 11 emits a laser beam 10 as excitationlight. The laser beam 10 is a divergent light beam, and the condenserlens 12 condenses the laser beam 10. The Ho³⁺-doped solid-state crystalis an Y₃Al₅O₁₂ crystal doped with 1 atomic percent (atm %) of Ho³⁺,which is referred to as Ho:YAG crystal.

The above elements 11 to 13 are fixed on a Peltier element 14, and athermistor 15 is attached to the Peltier element 14 for detectingtemperature. The output of the thermistor 15 is supplied to atemperature control circuit (not shown). Thus, the operation of thePeltier element 14 is controlled by the temperature control circuitbased on the output of the thermistor 15 so that the laser diode 11, thecondenser lens 12, and the Ho:YAG crystal 13 are maintained at apredetermined temperature.

The laser diode 11 is a broad-area type GaN-based compound laser diode,which oscillates at the wavelength of 420 nm.

In order to generate a laser beam having the wavelength of 550 nm by thetransition from ⁵S₂ to ⁵I₈ in the solid-state crystal 13, the backwardend surface (light entrance end face) 13 a of the solid-state crystal 13is coated to be highly reflective (HR) at the wavelength of 550 nm andantireflective (AR) at other wavelengths including 750 nm (thewavelength of the fluorescence generated by the transition from ⁵S₂ to⁵I₇) and 420 nm (the wavelength of the laser beam 10). On the otherhand, the forward end surface 13 b of the solid-state crystal 13 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 550 nm.

The laser beam 10 being emitted from the laser diode 11 and having thewavelength of 420 nm enters the solid-state crystal 13 through thebackward end surface 13 a. In the solid-state crystal 13, Ho³⁺ isexcited by the laser beam 10, and fluorescence having the wavelength of550 nm is generated by the transition from ⁵S₂ to ⁵I₈ in the solid-statecrystal 13. The fluorescence having the wavelength of 550 nm resonatesbetween the forward and backward end surfaces 13 b and 13 a, and causesa laser oscillation. Thus, a green laser beam 16 is generated in thesolid-state crystal 13, and output through the forward end surface 13 b.

In the laser-diode-excited solid-state laser apparatus as the firstembodiment, the applicants have obtained the green laser beam 16 withthe output power of 100 mW when the output power of the GaN-basedcompound laser diode 11 is 300 mW.

Alternatively, the coatings applied to the forward and backward endsurfaces 13 b and 13 a may be arranged so that a laser beam having thewavelength of 750 nm is obtained from the solid-state crystal 13, sincethe solid-state crystal 13 can generate the fluorescence having thewavelength of 750 nm by the transition from ⁵S₂ to ⁵I₇.

Second Embodiment

The second embodiment of the present invention is explained below. Sincethe construction of the laser-diode-excited solid-state laser apparatusas the second embodiment of the present invention has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the second embodiment.

The laser-diode-excited solid-state laser apparatus as the secondembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

That is, in the second embodiment, the solid-state crystal 13 is dopedwith 1 atomic percent (atm %) of Sm³⁺ instead of Ho³⁺. In addition, thelaser diode 11 used in the second embodiment emits a laser beam havingthe wavelength of 404 nm. Further, in order to generate a laser beamhaving the wavelength of 566 nm by the transition from ⁴G_(5/2) to⁶H_(5/2) in the solid-state crystal 13, the backward end surface (lightentrance end face) 13 a of the solid-state crystal 13 is coated to behighly reflective (HR) at the wavelength of 566 nm and antireflective(AR) at other wavelengths including 615 nm (the wavelength of thefluorescence generated by the transition from ⁴G_(5/2) to ⁶H_(7/2)), 650nm (the wavelength of the fluorescence generated by the transition from⁴F_(3/2) to ⁶H_(11/2)), and 404 nm (the wavelength of the excitationlaser beam 10). On the other hand, the forward end surface 13 b of thesolid-state crystal 13 is coated so as to have a transmittance of 1%(i.e., a reflectance of 99%) at the wavelength of 566 nm.

In the laser-diode-excited solid-state laser apparatus as the secondembodiment, the applicants have obtained a laser beam 16 having thewavelength of 566 nm and the output power of 40 mW when the output powerof the GaN-based compound laser diode 11 is 200 mW.

Alternatively, the coatings applied to the forward and backward endsurfaces 13 b and 13 a may be arranged so that a laser beam having awavelength of 615 or 650 nm is obtained from the Sm³⁺-doped solid-statecrystal 13, since the Sm³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 615 nm by the transition from⁴G_(5/2) to ⁶H_(7/2) and the fluorescence having the wavelength of 650nm by the transition from ⁴F_(3/2) to ⁶H_(11/2).

For example, in the case where the laser beam having the wavelength of615 nm is oscillated, it is possible to obtain a solid-state laser beam16 having the output power of 50 mW by using a GaN-based compound laserdiode 11 having the output power of 200 mW.

Third Embodiment

The third embodiment of the present invention is explained below. Sincethe construction of the laser-diode-excited solid-state laser apparatusas the third embodiment of the present invention also has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the third embodiment.

The laser-diode-excited solid-state laser apparatus as the thirdembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

That is, in the third embodiment, the solid-state crystal 13 is dopedwith 1 atomic percent (atm %) of Eu³⁺ instead of Ho³⁺. In addition, thelaser diode 11 used in the third embodiment emits a laser beam havingthe wavelength of 394 nm. Further, in order to generate a laser beamhaving the wavelength of 589 nm by the transition from ⁵D₀ to ⁷F₂ in thesolid-state crystal 13, the backward end surface (light entrance endface) 13 a of the solid-state crystal 13 is coated to be highlyreflective (HR) at the wavelength of 589 nm and antireflective (AR) atother wavelengths including wavelengths of fluorescence generated by theother transitions and the excitation wavelength of 394 nm. On the otherhand, the forward end surface 13 b of the solid-state crystal 13 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 589 nm.

In the laser-diode-excited solid-state laser apparatus as the thirdembodiment, the applicants have obtained a laser beam 16 having thewavelength of 589 nm and the output power of 20 mW when the output powerof the GaN-based compound laser diode 11 is 100 mW.

Fourth Embodiment

The fourth embodiment of the present invention is explained below. Sincethe construction of the laser-diode-excited solid-state laser apparatusas the fourth embodiment of the present invention has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the fourth embodiment.

The laser-diode-excited solid-state laser apparatus as the fourthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

That is, in the fourth embodiment, the solid-state crystal 13 is dopedwith 1 atomic percent (atm %) of Dy³⁺ instead of Ho³⁺. In addition, thelaser diode 11 used in the fourth embodiment emits a laser beam havingthe wavelength of 390 nm. Further, in order to generate a laser beamhaving the wavelength of 572 nm by the transition from ⁴F_(9/2) to⁶H_(13/2) in the solid-state crystal 13, the backward end surface (lightentrance end face) 13 a of the solid-state crystal 13 is coated to behighly reflective (HR) at the wavelength of 572 nm and antireflective(AR) at other wavelengths including 664 nm (the wavelength of thefluorescence generated by the transition from ⁴F_(9/2) to ⁶H_(11/2)) and390 nm (the wavelength of the excitation laser beam 10). On the otherhand, the forward end surface 13 b of the solid-state crystal 13 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 572 nm.

In the laser-diode-excited solid-state laser apparatus as the fourthembodiment, the applicants have obtained a laser beam 16 having thewavelength of 572 nm and the output power of 10 mW when the output powerof the GaN-based compound laser diode 11 is 100 mW.

Alternatively, the coatings applied to the forward and backward endsurfaces 13 b and 13 a may be arranged so that a laser beam having thewavelength of 664 nm is obtained from the Dy³⁺-doped solid-state crystal13, since the Dy³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 664 nm by the transition from⁴F_(9/2) to ⁶H_(11/2). In this case, it is possible to obtain asolid-state laser beam 16 having the output power of 10 mW by using aGaN-based compound laser diode 11 having the output power of 100 mW.

Fifth Embodiment

The fifth embodiment of the present invention is explained below. Sincethe construction of the laser-diode-excited solid-state laser apparatusas the fifth embodiment of the present invention has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the fifth embodiment.

The laser-diode-excited solid-state laser apparatus as the fifthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

That is, in the fifth embodiment, the solid-state crystal 13 is dopedwith 1 atomic percent (atm %) of Er³⁺ instead of Ho³⁺. In addition, thelaser diode 11 used in the fifth embodiment emits a laser beam havingthe wavelength of 406 nm. Further, in order to generate a laser beamhaving the wavelength of 554 nm by the transition from ²H_(9/2) to⁴I_(13/2) in the solid-state crystal 13, the backward end surface (lightentrance end face) 13 a of the solid-state crystal 13 is coated to behighly reflective (HR) at the wavelength of 554 nm and antireflective(AR) at other wavelengths including 540 nm (the wavelength of thefluorescence generated by the transition from ⁴S_(3/2) to ⁴I_(15/2)) and406 nm (the wavelength of the excitation laser beam 10). On the otherhand, the forward end surface 13 b of the solid-state crystal 13 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 554 nm.

In the laser-diode-excited solid-state laser apparatus as the fifthembodiment, the applicants have obtained a laser beam 16 having thewavelength of 554 nm and the output power of 30 mW when the output powerof the GaN-based compound laser diode 11 is 200 mW.

Alternatively, the coatings applied to the forward and backward endsurfaces 13 b and 13 a may be arranged so that a laser beam having thewavelength of 540 nm is obtained from the Er³⁺-doped solid-state crystal13, since the Er³⁺-doped solid-state crystal 13 can generate thefluorescence having the wavelength of 540 nm by the transition from⁴S_(3/2) to ⁴I_(15/2).

In addition, the excitation wavelength may be 380 nm instead of 406 nm.

Sixth Embodiment

The sixth embodiment of the present invention is explained below. Sincethe construction of the laser-diode-excited solid-state laser apparatusas the sixth embodiment of the present invention also has the sameconstruction as the first embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 1 are alsoreferred to in the following explanations of the sixth embodiment.

The laser-diode-excited solid-state laser apparatus as the sixthembodiment is different from the laser-diode-excited solid-state laserapparatus as the first embodiment in the rare-earth ion with which thesolid-state crystal 13 is doped and the coatings applied to the forwardand backward end surfaces 13 b and 13 a.

That is, in the sixth embodiment, the solid-state crystal 13 is dopedwith 1 atomic percent (atm %) of Tb³⁺ instead of Ho³⁺. In addition, thelaser diode 11 used in the sixth embodiment emits a laser beam havingthe wavelength of 380 nm. Further, in order to generate a laser beamhaving the wavelength of 540 nm by the transition from ⁵D₄ to ⁷F₅ in thesolid-state crystal 13, the backward end surface (light entrance endface) 13 a of the solid-state crystal 13 is coated to be highlyreflective (HR) at the wavelength of 540 nm and antireflective (AR) atother wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 380nm. On the other hand, the forward end surface 13 b of the solid-statecrystal 13 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 540 nm.

In the laser-diode-excited solid-state laser apparatus as the sixthembodiment, the applicants have obtained a laser beam 16 having thewavelength of 540 nm and the output power of 10 mW when the output powerof the GaN-based compound laser diode 11 is 100 mW.

Variations of First to Sixth Embodiments

Although the solid-state laser crystal 13 in the constructions of thefirst to sixth embodiments are an Y₃Al₅O₁₂ (YAG) crystal, alternatively,LiYF₄ (YLF), YVO₄, GdVO₄, BaY₂F₈, Ba(Y,Yb)₂F₈, LaF₃, Ca(NbO₃)₂, CaWO₄,SrMoO₄, YAlO₃(YAP), Y₂SiO₅, YP₅O₁₄, LaP₅O₁₄, LuAlO₃, LaCl₃, LaBr₃,PrBr₃, or the like may be used.

The active layers of the laser diodes used as excitation light sourcesin the constructions of the first to sixth embodiments can be made of anInGaN-based, InGaNAs-based, or GaNAs-based compound material. Inparticular, when an absorption band of a solid-state laser crystal islocated on the longer wavelength side of the output wavelength of thelaser diode, it is preferable to use the InGaNAs-based or GaNAs-basedcompound material, since the wavelength of the laser oscillation in theInGaNAs-based or GaNAs-based compound laser diode can be lengthened moreeasily than that in the InGaN-based compound laser diode. That is, theabsorption efficiency in the InGaNAs-based or GaNAs-based compound laserdiode can be enhanced more easily than that in the InGaN-based compoundlaser diode.

Seventh Embodiment

FIG. 2 is a side view illustrating an outline of the construction of alaser-diode-excited solid-state laser apparatus as the seventhembodiment of the present invention.

The laser-diode-excited solid-state laser apparatus of FIG. 2 comprisesa laser diode 111, a condenser lens 113, a solid-state laser medium 114,a resonator mirror 115, an optical wavelength conversion element 116,and an etalon 117. The laser diode 111 emits a laser beam 110 asexcitation light, where the laser beam 110 is a divergent light beam.The condenser lens 113 is realized by, for example, an indexdistribution type lens, and condenses the laser beam 110. Thesolid-state laser medium 114 is an Y₃Al₅O₁₂ crystal doped with 1 atomicpercent (atm %) of Ho³⁺, i.e., a Ho:YAG crystal. The resonator mirror115 is arranged on the forward side (i.e., on the right side in FIG. 2)of the Ho:YAG crystal 114. The optical wavelength conversion element 116and the etalon 117 are arranged between the Ho:YAG crystal 114 and theresonator mirror 115.

The above elements 114 to 117 are arranged on a common mount 130, whichis made of, for example, copper. The mount 130 is fixed on a Peltierelement 131, which constitutes a temperature adjustment means. The laserdiode 111 and the condenser lens 113 are respectively arranged on mounts132 and 133, which are made of, for example, copper. The mounts 132 and133 are also fixed on the Peltier element 131. The Peltier element 131is contained in a sealed case 136, which has a light exit window 135.

In addition, a thermistor 134 is attached to the mount 130. Theoperation of the Peltier element 131 is controlled by a temperaturecontrol circuit (not shown) based on a temperature detection signalwhich is output from the thermistor 134, so that the laser diode 111 andall of the elements constituting a solid-state laser resonator aremaintained at a predetermined common temperature, where the solid-statelaser resonator is formed by the Ho:YAG crystal 114 and the resonatormirror 115 as explained later.

The optical wavelength conversion element 116 is produced by forming aperiodic domain-inverted structure in a MgO-doped LiNbO₃ crystal, whichis a nonlinear optical material. In this example, the period of theperiodic domain-inverted structure is 2.0 micrometers, which is thefirst order period with respect to the wavelength (750 nm) of thefundamental harmonic and the wavelength (375 nm) of the second harmonic.The etalon 117 has a function of a wavelength selection element, and isprovided for realizing oscillation of the solid-state laser in a singlelongitudinal mode and reducing noise.

The laser diode 111 in the construction of FIG. 2 is a broad-area typelaser diode, which has an InGaN active layer and oscillates at thewavelength of 420 nm.

The backward end surface 114 a of the Ho:YAG crystal 114 is a lightentrance surface, and is coated so as to have the followingtransmittance and reflectances. That is, the transmittance of thebackward end surface 114 a at the wavelength of 420 nm is 80% or higher,so that light having the wavelength of 420 nm efficiently transmitsthrough the backward end surface 114 a. In addition, the reflectance ofthe backward end surface 114 a at the wavelength of 750 nm is high,where the wavelength of 750 nm corresponds to one of the oscillationpeaks of Ho³⁺. For example, the reflectance of the backward end surface114 a at the wavelength of 750 nm is 99% or higher, and preferably 99.9%or higher. Further, the reflectances of the backward end surface 114 aat the wavelengths of the other oscillation peaks of Ho³⁺ (i.e., at thewavelengths of 550 nm, 980 nm, 1,010 nm, and 1,210 nm) are low. Forexample, the reflectances at the wavelengths of 550 nm, 980 nm, 1,010nm, and 1,210 nm are 60% or lower, and preferably 30% or lower.

On the other hand, the forward end surface 114 b of the Ho:YAG crystal114 is coated so as to have a low reflectance (e.g., 0.2% or lower) atthe wavelength of 750 nm (the wavelength of the fundamental harmonic)and a high reflectance (e.g., 95% or higher) at the wavelength of 375 nm(the wavelength of the second harmonic).

The mirror surface 115 a of the resonator mirror 115 is coated so as tohave a high reflectance (e.g., 99% or higher, and preferably 99.9% orhigher) at the wavelength of 750 nm, a transmittance of 95% at thewavelength of 375 nm, and low reflectances (e.g., 60% or lower, andpreferably 30% or lower) at the wavelengths of the other oscillationpeaks including the wavelengths of 550 nm, 980 nm, 1,010 nm, and 1,210nm.

The laser beam 110 emitted from the laser diode 111 has the wavelengthof 420 nm, and enters the Ho:YAG crystal 114 through the backward endsurface 114 a. Since Ho³⁺ in the Ho:YAG crystal 114 is excited by thelaser beam 110, the Ho:YAG crystal 114 generates light having thewavelength of 750 nm by the transition from ⁵S₂ to ⁵I₇. Then, laseroscillation at the wavelength of 750 nm occurs in a resonator which isformed by the backward end surface 114 a of the Ho:YAG crystal 114 andthe mirror surface 115 a of the resonator mirror 115, and a solid-statelaser beam 120 having the wavelength of 750 nm is generated. Thesolid-state laser beam 120 enters the optical wavelength conversionelement 116, and is converted to a second harmonic 121 having thewavelength of 375 nm, which is one-half of the wavelength of thesolid-state laser beam 120.

Since the mirror surface 115 a of the resonator mirror 115 is coated asdescribed before, only the second harmonic 121 exits through theresonator mirror 115. Thus, the second harmonic 121 exits from thesealed case 136 through the light exit window 135.

Since, in the laser-diode-excited solid-state laser apparatus as theseventh embodiment of the present invention, the Ho:YAG crystal 114 isexcited with the InGaN laser diode, the efficiency and the output powerare enhanced for the reason explained before. Actually, the applicantshave obtained the second harmonic 121 with the output power of 40 mWwhen the output power of the laser diode 111 is 300 mW.

Although the operation explained above corresponds to the so-calledcontinuous wave (CW) operation, the efficiency of the wavelengthconversion can be enhanced by inserting a Q switch element into theresonator. In this case, the laser-diode-excited solid-state laserapparatus operates in a pulse mode. Alternatively, pulsed ultravioletlight can be obtained with high efficiency and high output power bydriving the excitation laser diode in a pulse mode since the COD(catastrophic optical damage) thresholds of the GaN-based compound laserdiodes are high.

Further, the solid-state crystal 114 can generate a solid-state laserbeam having the wavelength of 550 nm by the transition from ⁵S₂ to ⁵I₈.Therefore, when this solid-state laser beam is converted into a secondharmonic by using the optical wavelength conversion element 116, it ispossible to obtain ultraviolet light having the wavelength of 275 nmwith a high intensity.

Eighth Embodiment

The construction of the laser-diode-excited solid-state laser apparatusas the eighth embodiment of the present invention also has the sameconstruction as the seventh embodiment except for the following portionsof the construction.

In the eighth embodiment, the solid-state laser crystal 114 is dopedwith Sm³⁺ instead of Ho³⁺, and the laser beam 110 emitted from the laserdiode 111 has a wavelength of 404 nm, so that the solid-state lasercrystal 114 generates fluorescence having a wavelength of about 566,615, or 650 nm. In addition, the coatings applied to the backward endsurface 114 a and the forward end surface 114 b of the solid-state lasercrystal 114 and the mirror surface 115 a of the resonator mirror 115 arearranged so that a solid-state laser beam having the wavelength of about566, 615, or 650 nm is generated in the resonator, a second harmonic 121having the wavelength of about 283, 308, or 325 nm is generated by theoptical wavelength conversion element 116, and ultraviolet light havingthe wavelength of about 283, 308, or 325 nm is output through theresonator mirror 115.

In the case where the Sm³⁺-doped solid-state laser crystal 114 is used,as well as the case where the Ho³⁺-doped solid-state laser crystal 114is used, particularly high output power can be obtained.

Ninth Embodiment

The construction of the laser-diode-excited solid-state laser apparatusas the ninth embodiment of the present invention also has the sameconstruction as the seventh embodiment except for the following portionsof the construction.

In the ninth embodiment, the solid-state laser crystal 114 is doped withEu³⁺ instead of Ho³⁺, and the laser beam 110 emitted from the laserdiode 111 has a wavelength of 394 nm, so that the solid-state lasercrystal 114 generates fluorescence having the wavelength of about 589nm. In addition, the coatings applied to the backward end surface 114 aand the forward end surface 114 b of the solid-state laser crystal 114and the mirror surface 115 a of the resonator mirror 115 are arranged sothat a solid-state laser beam having the wavelength of about 589 nm isgenerated in the resonator, a second harmonic 121 having the wavelengthof about 295 nm is generated by the optical wavelength conversionelement 116, and ultraviolet light having the wavelength of about 295 nmis output through the resonator mirror 115.

Tenth Embodiment

The construction of the laser-diode-excited solid-state laser apparatusas the tenth embodiment of the present invention also has the sameconstruction as the seventh embodiment except for the following portionsof the construction.

In the tenth embodiment, the solid-state laser crystal 114 is doped withDy³⁺ instead of Ho³⁺, and the laser beam 110 emitted from the laserdiode 111 has a wavelength of 390 nm, so that the solid-state lasercrystal 114 generates fluorescence having a wavelength of about 572 or664 nm. In addition, the coatings applied to the backward end surface114 a and the forward end surface 114 b of the solid-state laser crystal114 and the mirror surface 115 a of the resonator mirror 115 arearranged so that a solid-state laser beam having the wavelength of about572 or 664 nm is generated in the resonator, a second harmonic 121having the wavelength of about 286 or 332 nm is generated by the opticalwavelength conversion element 116, and ultraviolet light having thewavelength of about 286 or 332 nm is output through the resonator mirror115.

In the case where the Dy³⁺-doped solid-state laser crystal 114 is used,and the solid-state laser beam having the wavelength of 664 nm isgenerated, particularly high output power can be obtained.

Eleventh Embodiment

The construction of the laser-diode-excited solid-state laser apparatusas the eleventh embodiment of the present invention also has the sameconstruction as the seventh embodiment except for the following portionsof the construction.

In the eleventh embodiment, the solid-state laser crystal 114 is dopedwith Er³⁺ instead of Ho³⁺, and the laser beam 110 emitted from the laserdiode 111 has a wavelength of 406 or 380 nm, so that the solid-statelaser crystal 114 generates fluorescence having a wavelength of about540 or 554 nm. In addition, the coatings applied to the backward endsurface 114 a and the forward end surface 114 b of the solid-state lasercrystal 114 and the mirror surface 115 a of the resonator mirror 115 arearranged so that a solid-state laser beam having the wavelength of about540 or 554 nm is generated in the resonator, a second harmonic 121having the wavelength of about 270 or 277 nm is generated by the opticalwavelength conversion element 116, and ultraviolet light having thewavelength of about 270 or 277 nm is output through the resonator mirror115.

Variations of Seventh to Eleventh Embodiments

Although the active layers of the laser diodes used as excitation lightsources in the constructions of the seventh to eleventh embodiments aremade of InGaN, alternatively, the active layers of the laser diodes maybe made of an InGaNAs-based or GaNAs-based compound material.

Although the solid-state laser crystal 114 in the constructions of theseventh to eleventh embodiments are an Y₃Al₅O₁₂ (YAG) crystal,alternatively, BaY₂F₈, Ba(Y,Yb)₂F₈, LaF₃, Ca(NbO₃)₂, CaWO₄, SrMoO₄,YAlO₃ (YAP), LiYF₄ (YLF), Y₂SiO₅, YP₅O₁₄, LaP₅O₁₄, LuAlO₃, LaCl₃, LaBr₃,PrBr₃, or the like may be used.

The period of the periodic domain-inverted structure in the opticalwavelength conversion element 116 may not be necessarily the first orderperiod with respect to the wavelength of the fundamental harmonic.Alternatively, the second or third order period may be used. Forexample, the third order period with respect to the wavelength of 750 nmis 6.0 micrometers.

In addition, the optical wavelength conversion element 116 may not be atype which has the periodic domain-inverted structure. Alternatively,the optical wavelength conversion element 116 may be a type which ismade of B—BaBO₃, LBO, CLBO, GdYCOB, YCOB, or the like.

Further, the excitation laser diodes 111 may not be the broad-area type.Alternatively, the laser diodes 111 may be a type which includes a MOPA(master oscillator power amplifier) or α-DFB (distributed feedback)structure.

Twelfth Embodiment

FIG. 3 is a side view illustrating an outline of the construction of afiber laser apparatus as the twelfth embodiment of the presentinvention.

The fiber laser apparatus of FIG. 3 comprises a laser diode 211, acondenser lens 212, and an optical fiber 213. The laser diode 211 emitsa laser beam 210 as excitation light, where the laser beam 210 is adivergent light beam. The condenser lens 212 condenses the laser beam210. The optical fiber 213 has a core 220 which is doped with Ho³⁺.

The laser diode 211 in the construction of FIG. 3 is a broad-area typelaser diode, which has an active layer made of a GaN-based compoundmaterial, and oscillates at the wavelength of 420 nm.

FIG. 4 shows a cross section of the optical fiber 213 used in the fiberlaser apparatus of FIG. 3. As illustrated in FIG. 4, the optical fiber213 comprises the core 220 and first and second claddings 221 and 222.The first cladding 221 is arranged around the core 220, and the secondcladding 222 is arranged around the first cladding 221. Thecross-sectional shape of each of the core 220 and the second cladding222 is a true circle, and the cross-sectional shape of the firstcladding 221 is nearly a rectangle.

The core 220 is made of a zirconium-based fluoride glass, e.g., ZBLANP(ZrF₄—BaF₂—LaF₃—AlF₃—AlF₃—NaF—PbF₂), and doped with 1 atomic percent(atm %) of Ho³⁺. The first cladding 221 is made of, for example, ZBLAN(ZrF₄—BaF₂—LaF₃—AlF₃—NaF), and the second cladding 222 is made of, forexample, a polymer. Alternatively, the core 220 may be made of ZBLAN orindium-gallium-based fluoride glass. For example, the core 220 may bemade of IGPZCL, i.e., (InF₃—GaF₃—LaF₃)—(PbF₂—ZnF₂)—CdF, or the like.

The laser beam 210 condensed by the condenser lens 212 enters the firstcladding 221 of the optical fiber 213, and propagates through the firstcladding 221 in a guided mode. That is, the first cladding 221 behavesas a core for the laser beam 210. During the propagation, the laser beam210 also passes through the core 220. In the core 220, Ho³⁺ is excitedby the laser beam 210, so that fluorescence having the wavelength of 550nm is generated by the transition from ⁵S₂ to ⁵I₈. The fluorescencepropagates through the core 220 in a guided mode.

In addition, the core 220 made of ZBLANP can also generate otherfluorescence such as the fluorescence having the wavelength of 750 nm,which is generated by the transition from ⁵S₂ to ⁵I₇. Therefore, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 550 nm, andantireflective (AR) at the excitation wavelength of 420 nm and thewavelengths of the fluorescence other than 550 nm. In addition, thelight exit end surface 213 b of the optical fiber 213 is coated so as totransmit only 1% of the light having the wavelength of 550 nm.

In the above configuration, the above fluorescence having the wavelengthof 550 nm resonates between the light entrance end surface 213 a and thelight exit end surface 213 b of the optical fiber 213, i.e., laseroscillation occurs at the wavelength of 550 nm. Thus, a green laser beam215 having the wavelength of 550 nm is generated in the optical fiber213, and exits from the light exit end surface 213 b of the opticalfiber 213 to the forward side of the fiber laser apparatus of FIG. 3.

In this example, the laser beam 215 propagates through the core 220 in asingle mode, and the laser beam 210 propagates through the firstcladding 221 in multiple modes. Therefore, it is possible to use ahigh-power, broad-area type laser diode as the excitation light source,and input the laser beam 210 from the high-power, broad-area type laserdiode into the optical fiber 213 with high coupling efficiency.

In addition, since the cross-sectional shape of the first cladding 221is nearly a rectangle, the laser beam 210 propagates through irregularreflection paths within the first cladding 221, and therefore theprobability of entrance of the laser beam 210 into the core 220 isenhanced.

Further, since the wavelength 420 nm of the laser beam 210 is within thewavelength range in which the output power of the GaN-based compoundlaser diodes is enhanced, the amount of the laser beam 210 absorbed bythe optical fiber 213 becomes great, and high efficiency and high outputpower can be achieved. Actually, the applicants have obtained the greenlaser beam 215 with the output power of 150 mW when the output power ofthe laser diode 211 is 300 mW, and the length of the optical fiber 213is 1 m.

Furthermore, the Ho³⁺-doped core 220 of the optical fiber 213 cangenerate the fluorescence having the wavelength of 750 nm by thetransition from ⁵S₂ to ⁵I₇. Therefore, when the coatings applied to thelight entrance end surface 213 a and the light exit end surface 213 b ofthe optical fiber 213 are appropriately arranged, the fiber laserapparatus of FIG. 3 can emit a laser beam having the wavelength of 750nm.

Thirteenth Embodiment

The thirteenth embodiment of the present invention is explained below.Since the construction of the fiber laser apparatus as the thirteenthembodiment of the present invention has the same construction as thetwelfth embodiment except for the portions of the construction explainedbelow, the reference numerals in FIG. 3 are also referred to in thefollowing explanations of the thirteenth embodiment.

The fiber laser apparatus as the thirteenth embodiment is different fromthe fiber laser apparatus as the twelfth embodiment in the rare-earthion with which the core 220 of the optical fiber 213 is doped and thecoatings applied to the light entrance end surface 213 a and the lightexit end surface 213 b of the optical fiber 213.

That is, in the thirteenth embodiment, the core 220 of the optical fiber213 is doped with 1 atomic percent (atm %) of Sm³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the thirteenth embodiment emits alaser beam having the wavelength of 404 nm. Further, in order togenerate a laser beam having the wavelength of 566 nm by the transitionfrom ⁴G_(5/2) to ⁶H_(5/2) in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 566 nm and antireflective(AR) at other wavelengths including 615 nm (the wavelength of thefluorescence generated by the transition from ⁴G_(5/2) to ⁶H_(7/2)), 650nm (the wavelength of the fluorescence generated by the transition from⁴F_(3/2) to ⁶H_(11/2)), and 404 nm (the wavelength of the excitationlaser beam 210). On the other hand, the light exit end surface 213 b ofthe optical fiber 213 is coated so as to have a transmittance of 1%(i.e., a reflectance of 99%) at the wavelength of 566 nm.

In the fiber laser apparatus as the thirteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 566nm and the output power of 110 mW when the output power of the GaN-basedcompound laser diode 211 is 200 mW, and the length of the optical fiber213 is 1 m.

Alternatively, the coatings applied to the light entrance end surface213 a and the light exit end surface 213 b of the optical fiber 213 maybe arranged so that a laser beam having a wavelength of 615 or 650 nm isgenerated in the optical fiber 213, since the Sm³⁺-doped core 220 of theoptical fiber 213 can generate the fluorescence having the wavelength of615 nm by the transition from ⁴G_(5/2) to ⁶H_(7/2) and the fluorescencehaving the wavelength of 650 nm by the transition from ⁴F_(3/2) to⁶H_(11/2).

Fourteenth Embodiment

The fourteenth embodiment of the present invention is explained below.Since the construction of the fiber laser apparatus as the fourteenthembodiment of the present invention also has the same construction asthe twelfth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 3 are also referred toin the following explanations of the fourteenth embodiment.

The fiber laser apparatus as the fourteenth embodiment is different fromthe fiber laser apparatus as the twelfth embodiment in the rare-earthion with which the core 220 of the optical fiber 213 is doped and thecoatings applied to the light entrance end surface 213 a and the lightexit end surface 213 b of the optical fiber 213.

That is, in the fourteenth embodiment, the core 220 of the optical fiber213 is doped with 1 atomic percent (atm %) of Eu³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the fourteenth embodiment emits alaser beam having the wavelength of 394 nm. Further, in order togenerate a laser beam having the wavelength of 589 nm by the transitionfrom ⁵D₀ to ⁷F₂ in the core 220 of the optical fiber 213, the lightentrance end surface 213 a of the optical fiber 213 is coated to behighly reflective (HR) at the wavelength of 589 nm and antireflective(AR) at other wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 394nm. On the other hand, the light exit end surface 213 b of the opticalfiber 213 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 589 nm.

In the fiber laser apparatus as the fourteenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 589nm and the output power of 40 mW when the output power of the GaN-basedcompound laser diode 211 is 100 mW, and the length of the optical fiber213 is 1 m.

Fifteenth Embodiment

The fifteenth embodiment of the present invention is explained below.Since the construction of the fiber laser apparatus as the fifteenthembodiment of the present invention has the same construction as thetwelfth embodiment except for the portions of the construction explainedbelow, the reference numerals in FIG. 3 are also referred to in thefollowing explanations of the fifteenth embodiment.

The fiber laser apparatus as the fifteenth embodiment is different fromthe fiber laser apparatus as the twelfth embodiment in the rare-earthion with which the core 220 of the optical fiber 213 is doped and thecoatings applied to the light entrance end surface 213 a and the lightexit end surface 213 b of the optical fiber 213.

That is, in the fifteenth embodiment, the core 220 of the optical fiber213 is doped with 1 atomic percent (atm %) of Dy³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the fifteenth embodiment emits alaser beam having the wavelength of 390 nm. Further, in order togenerate a laser beam having the wavelength of 572 nm by the transitionfrom ⁴F_(9/2) to ⁶H_(13/2) in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 572 nm and antireflective(AR) at other wavelengths including 664 nm (the wavelength of thefluorescence generated by the transition from ⁴F_(9/2) to ⁶H_(11/2)) and390 nm (the wavelength of the excitation laser beam 210). On the otherhand, the light exit end surface 213 b of the optical fiber 213 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 572 nm.

In the fiber laser apparatus as the fifteenth embodiment, the applicantshave obtained a laser beam 215 having the wavelength of 572 nm and theoutput power of 50 mW when the output power of the GaN-based compoundlaser diode 211 is 100 mW, and the length of the optical fiber 213 is 1m.

Alternatively, the coatings applied to the light entrance end surface213 a and the light exit end surface 213 b of the optical fiber 213 maybe arranged so that a laser beam having the wavelength of 664 nm isgenerated in the optical fiber 213, since the Dy³⁺-doped core 220 of theoptical fiber 213 can generate the fluorescence having the wavelength of664 nm by the transition from ⁴F_(9/2) to ⁶H_(11/2).

Sixteenth Embodiment

The sixteenth embodiment of the present invention is explained below.Since the construction of the fiber laser apparatus as the sixteenthembodiment of the present invention has the same construction as thetwelfth embodiment except for the portions of the construction explainedbelow, the reference numerals in FIG. 3 are also referred to in thefollowing explanations of the sixteenth embodiment.

The fiber laser apparatus as the sixteenth embodiment is different fromthe fiber laser apparatus as the twelfth embodiment in the rare-earthion with which the core 220 of the optical fiber 213 is doped and thecoatings applied to the light entrance end surface 213 a and the lightexit end surface 213 b of the optical fiber 213.

That is, in the sixteenth embodiment, the core 220 of the optical fiber213 is doped with 1 atomic percent (atm %) of Er³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the sixteenth embodiment emits alaser beam having the wavelength of 406 nm. Further, in order togenerate a laser beam having the wavelength of 554 nm by the transitionfrom ²H_(9/2) to ⁴I_(13/2) in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 554 nm and antireflective(AR) at other wavelengths including 540 nm (the wavelength of thefluorescence generated by the transition from ⁴S_(3/2) to ⁴I_(15/2)) and406 nm (the wavelength of the excitation laser beam 210). On the otherhand, the light exit end surface 213 b of the optical fiber 213 iscoated so as to have a transmittance of 1% (i.e., a reflectance of 99%)at the wavelength of 554 nm.

In the fiber laser apparatus as the sixteenth embodiment, the applicantshave obtained a laser beam 215 having the wavelength of 554 nm and theoutput power of 120 mW when the output power of the GaN-based compoundlaser diode 211 is 200 mW, and the length of the optical fiber 213 is 1m.

Alternatively, the coatings applied to the light entrance end surface213 a and the light exit end surface 213 b of the optical fiber 213 maybe arranged so that a laser beam having the wavelength of 540 nm isgenerated in the optical fiber 213, since the Er³⁺-doped core 220 of theoptical fiber 213 can generate the fluorescence having the wavelength of540 nm by the transition from ⁴S_(3/2) to ⁴I_(15/2).

In addition, the excitation wavelength may be 380 nm instead of 406 nm.

Seventeenth Embodiment

The seventeenth embodiment of the present invention is explained below.Since the construction of the fiber laser apparatus as the seventeenthembodiment of the present invention also has the same construction asthe twelfth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 3 are also referred toin the following explanations of the seventeenth embodiment.

The fiber laser apparatus as the seventeenth embodiment is differentfrom the fiber laser apparatus as the twelfth embodiment in therare-earth ion with which the core 220 of the optical fiber 213 is dopedand the coatings applied to the light entrance end surface 213 a and thelight exit end surface 213 b of the optical fiber 213.

That is, in the seventeenth embodiment, the core 220 of the opticalfiber 213 is doped with 1 atomic percent (atm %) of Tb³⁺ instead ofHo³⁺. In addition, the laser diode 211 used in the seventeenthembodiment emits a laser beam having the wavelength of 380 nm. Further,in order to generate a laser beam having the wavelength of 540 nm by thetransition from ⁵D₄ to ⁷F₅ in the core 220 of the optical fiber 213, thelight entrance end surface 213 a of the optical fiber 213 is coated tobe highly reflective (HR) at the wavelength of 540 nm and antireflective(AR) at other wavelengths including the wavelengths of the fluorescencegenerated by the other transitions and the excitation wavelength of 380nm. On the other hand, the light exit end surface 213 b of the opticalfiber 213 is coated so as to have a transmittance of 1% (i.e., areflectance of 99%) at the wavelength of 540 nm.

In the fiber laser apparatus as the seventeenth embodiment, theapplicants have obtained a laser beam 215 having the wavelength of 540nm and the output power of 30 mW when the output power of the GaN-basedcompound laser diode 211 is 100 mW, and the length of the optical fiber213 is 1 m.

Eighteenth Embodiment

FIG. 5 is a side view illustrating an outline of the construction of afiber laser amplifier as the eighteenth embodiment of the presentinvention. In FIG. 5, elements having the same reference numbers as FIG.3 have the same functions as the corresponding elements in FIG. 3.

The fiber laser amplifier of FIG. 5 comprises a laser diode 211, acollimator lens 250, a condenser lens 251, a beam splitter 252, anoptical fiber 253, an SHG (second harmonic generation) laser unit 256,and another collimator lens 257. The laser diode 211 emits a laser beam210 having the wavelength of 420 nm as excitation light, where the laserbeam 210 is a divergent light beam. The collimator lens 250 collimatesthe laser beam 210. The beam splitter 252 is arranged between thecollimator lens 250 and the condenser lens 251, and the laser beam 210collimated by the collimator lens 250 passes through the beam splitter252. The condenser lens 251 condenses the collimated laser beam 210, andthe condensed laser beam 210 enters the optical fiber 253, which has aHo³⁺-doped core.

The SHG laser unit 256 is provided for emitting a laser beam 255 havingthe wavelength of 550 nm. Although not shown, the SHG laser unit 256includes a DBR (distributed Bragg reflection) type laser diode and anoptical waveguide. The DBR type laser diode is provided as a lightsource of a fundamental wave, and emits a laser beam having thewavelength of 1,100 nm. The optical waveguide is provided as awavelength conversion element, which is made of a nonlinear opticalmaterial and has periodic domain-inverted structure. In the SHG laserunit 256, the wavelength of the laser beam emitted from the DBR typelaser diode is reduced in half by the optical waveguide, and the laserbeam 255 having the wavelength of 550 nm is generated.

The laser beam 255 is collimated by the collimator lens 257, and thecollimated laser beam 255 enters the beam splitter 252, in which thecollimated laser beam 255 is reflected toward the condenser lens 251.Then, the collimated and reflected laser beam 255 is condensed by thecondenser lens 251, and enters the optical fiber 253 together with thelaser beam 210 from the laser diode 211.

The optical fiber 253 has basically the same construction as the opticalfiber 213 in the twelfth embodiment, except that the light entrance endsurface 253 a and the light exit end surface 253 b of the optical fiber253 are coated to be antireflective (AR) at the above wavelengths of 420nm and 550 nm.

The laser beam 210 excites Ho³⁺ in the core of the optical fiber 253,and fluorescence having the wavelength of 550 nm is generated by theexcitation of Ho³⁺, in the same manner as the twelfth embodiment. Sincethe wavelength of the above fluorescence is the same as the wavelengthof the laser beam 255 from the SHG laser unit 256, the laser beam 255 isamplified in the optical fiber 253 by receiving the energy of thefluorescence, and the amplified laser beam 255′ is output through thelight exit end surface 253 b to the forward side of the fiber laseramplifier of FIG. 5.

Actually, the applicants have obtained the amplified laser beam 255′with the output power of 60 mW when the output power of the SHG laserunit 256 is 1 mW.

In addition, when a function of modulation of the laser beam 255 isprovided to the DBR type laser diode in the SHG laser unit 256, it ispossible to modulate the laser beam 255′.

Further, the Ho³⁺-doped core of the optical fiber 253 can generate thefluorescence having the wavelength of 750 nm by the transition from ⁵S₂to ⁵I₇. Therefore, when the coatings applied to the light entrance endsurface 253 a and the light exit end surface 253 b of the optical fiber253 are appropriately arranged, the fiber laser amplifier as theeighteenth embodiment of the present invention can amplify a laser beamhaving the wavelength of 750 nm.

Nineteenth Embodiment

The nineteenth embodiment of the present invention is explained below.Since the construction of the fiber laser amplifier as the nineteenthembodiment of the present invention has the same construction as theeighteenth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 5 are also referred toin the following explanations of the nineteenth embodiment.

The fiber laser amplifier as the nineteenth embodiment is different fromthe fiber laser amplifier as the eighteenth embodiment in the rare-earthion with which the core of the optical fiber 253 is doped and thecoatings applied to the light entrance end surface 253 a and the lightexit end surface 253 b of the optical fiber 253.

That is, in the nineteenth embodiment, the core of the optical fiber 253is doped with 1 atomic percent (atm %) of Sm³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the nineteenth embodiment emits alaser beam having the wavelength of 404 nm. Further, the light entranceend surface 253 a and the light exit end surface 253 b of the opticalfiber 253 are coated to be antireflective (AR) at the wavelengths of 566nm (the wavelength of the fluorescence generated by the transition from⁴G_(5/2) to ⁶H_(5/2) in the core of the optical fiber 253) and 404 nm(the wavelength of the excitation laser beam 210).

In the fiber laser amplifier as the nineteenth embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 100 mW when the output power of theSHG laser unit 256 is 1.5 mW.

Alternatively, the coatings applied to the light entrance end surface253 a and the light exit end surface 253 b of the optical fiber 253 maybe arranged so that a laser beam having a wavelength of 615 or 650 nm isamplified in the optical fiber 253, since the Sm³⁺-doped core of theoptical fiber 253 can generate the fluorescence having the wavelength of615 nm by the transition from ⁴G_(5/2) to ⁶H_(7/2) and the fluorescencehaving the wavelength of 650 nm by the transition from ⁴F_(3/2) to⁶H_(1/2).

Twentieth Embodiment

The twentieth embodiment of the present invention is explained below.Since the construction of the fiber laser amplifier as the twentiethembodiment of the present invention also has the same construction asthe eighteenth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 5 are also referred toin the following explanations of the twentieth embodiment.

The fiber laser amplifier as the twentieth embodiment is different fromthe fiber laser amplifier as the eighteenth embodiment in the rare-earthion with which the core of the optical fiber 253 is doped and thecoatings applied to the light entrance end surface 253 a and the lightexit end surface 253 b of the optical fiber 253.

That is, in the twentieth embodiment, the core of the optical fiber 253is doped with 1 atomic percent (atm %) of Eu³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the twentieth embodiment emits alaser beam having the wavelength of 394 nm. Further, the light entranceend surface 253 a and the light exit end surface 253 b of the opticalfiber 253 are coated to be antireflective (AR) at the wavelengths of 589nm (the wavelength of the fluorescence generated by the transition from⁵D₀ to ⁷F₂ in the core of the optical fiber 253) and 394 nm (thewavelength of the excitation laser beam 210).

In the fiber laser amplifier as the twentieth embodiment, the applicantshave obtained the amplified laser beam 255′ from the optical fiber 253with the output power of 50 mW when the output power of the SHG laserunit 256 is 1 mW.

Twenty-First Embodiment

The twenty-first embodiment of the present invention is explained below.Since the construction of the fiber laser amplifier as the twenty-firstembodiment of the present invention has the same construction as theeighteenth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 5 are also referred toin the following explanations of the twenty-first embodiment.

The fiber laser amplifier as the twenty-first embodiment is differentfrom the fiber laser amplifier as the eighteenth embodiment in therare-earth ion with which the core of the optical fiber 253 is doped andthe coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

That is, in the twenty-first embodiment, the core of the optical fiber253 is doped with 1 atomic percent (atm %) of Dy³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the twenty-first embodiment emitsa laser beam having the wavelength of 390 nm. Further, the lightentrance end surface 253 a and the light exit end surface 253 b of theoptical fiber 253 are coated to be antireflective (AR) at thewavelengths of 572 nm (the wavelength of the fluorescence generated bythe transition from ⁴F_(9/2) to ⁶H_(13/2) in the core of the opticalfiber 253) and 390 nm (the wavelength of the excitation laser beam 210).

In the fiber laser amplifier as the twenty-first embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 80 mW when the output power of theSHG laser unit 256 is 1.5 mW.

Alternatively, the coatings applied to the light entrance end surface253 a and the light exit end surface 253 b of the optical fiber 253 maybe arranged so that a laser beam having the wavelength of 664 nm isamplified in the optical fiber 253, since the Dy³⁺-doped core of theoptical fiber 253 can generate the fluorescence having the wavelength of664 nm by the transition from ⁴F_(9/2) to ⁶H_(11/2).

Twenty-Second Embodiment

The twenty-second embodiment of the present invention is explainedbelow. Since the construction of the fiber laser amplifier as thetwenty-second embodiment of the present invention has the sameconstruction as the eighteenth embodiment except for the portions of theconstruction explained below, the reference numerals in FIG. 5 are alsoreferred to in the following explanations of the twenty-secondembodiment.

The fiber laser amplifier as the twenty-second embodiment is differentfrom the fiber laser amplifier as the eighteenth embodiment in therare-earth ion with which the core of the optical fiber 253 is doped andthe coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

That is, in the twenty-second embodiment, the core of the optical fiber253 is doped with 1 atomic percent (atm%) of Er³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the twenty-second embodiment emitsa laser beam having the wavelength of 406 nm. Further, the lightentrance end surface 253 a and the light exit end surface 253 b of theoptical fiber 253 are coated to be antireflective (AR) at thewavelengths of 554 nm (the wavelength of the fluorescence generated bythe transition from ²H_(9/2) to ⁴I_(13/2) in the core of the opticalfiber 253) and 406 nm (the wavelength of the excitation laser beam 210).

In the fiber laser amplifier as the twenty-second embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 80 mW when the output power of theSHG laser unit 256 is 1 mW.

Alternatively, the coatings applied to the light entrance end surface253 a and the light exit end surface 253 b of the optical fiber 253 maybe arranged so that a laser beam having the wavelength of 540 nm isamplified in the optical fiber 253, since the Er³⁺-doped core of theoptical fiber 253 can generate the fluorescence having the wavelength of540 nm by the transition from ⁴S_(3/2) to ⁴I_(15/2).

In addition, the excitation wavelength may be 380 nm instead of 406 nm.

Twenty-Third Embodiment

The twenty-third embodiment of the present invention is explained below.Since the construction of the fiber laser amplifier as the twenty-thirdembodiment of the present invention also has the same construction asthe eighteenth embodiment except for the portions of the constructionexplained below, the reference numerals in FIG. 5 are also referred toin the following explanations of the twenty-third embodiment.

The fiber laser amplifier as the twenty-third embodiment is differentfrom the fiber laser amplifier as the eighteenth embodiment in therare-earth ion with which the core of the optical fiber 253 is doped andthe coatings applied to the light entrance end surface 253 a and thelight exit end surface 253 b of the optical fiber 253.

That is, in the twenty-third embodiment, the core of the optical fiber253 is doped with 1 atomic percent (atm %) of Tb³⁺ instead of Ho³⁺. Inaddition, the laser diode 211 used in the twenty-third embodiment emitsa laser beam having the wavelength of 380 nm. Further, the lightentrance end surface 253 a and the light exit end surface 253 bof theoptical fiber 253 are coated to be antireflective (AR) at thewavelengths of 540 nm (the wavelength of the fluorescence generated bythe transition from ⁵D₄ to ⁷F₅ in the core of the optical fiber 253) and380 nm (the wavelength of the excitation laser beam 210).

In the fiber laser amplifier as the twenty-third embodiment, theapplicants have obtained the amplified laser beam 255′ from the opticalfiber 253 with the output power of 70 mW when the output power of theSHG laser unit 256 is 1.5 mW.

Variations of Twelfth to Twenty-third Embodiments

The active layers of the laser diodes used as excitation light sourcesin the constructions of the twelfth to twenty-third embodiments can bemade of an InGaN-based, InGaNAs-based, or GaNAs-based compound material.In particular, when an absorption band of a solid-state laser crystal islocated on the longer wavelength side of the output wavelength of thelaser diode, it is preferable to use the InGaNAs-based or GaNAs-basedcompound material, since the wavelength of the laser oscillation in theInGaNAs-based or GaNAs-based compound laser diode can be lengthened moreeasily than that in the InGaN-based compound laser diode. That is, theabsorption efficiency in the InGaNAs-based or GaNAs-based compound laserdiode can be enhanced more easily than that in the InGaN-based compoundlaser diode.

1. A laser-diode-excited solid-state laser apparatus comprising: aGaN-based compound laser diode which emits an excitation laser beam; anda solid-state laser crystal which is doped with Tb³⁺, and emits asolid-state laser beam generated by a transition from ⁵D₄ to ⁷F₅ whenthe solid-state laser crystal is excited with said excitation laserbeam, wherein the solid-state laser crystal is a bulk structure YAGsolid-state crystal; the solid-state laser crystal includes a firstcoating disposed on a backward end surface of the solid-state crystaland a second coating disposed on a forward end surface of thesolid-state laser crystal, which enable the solid-state crystal to emitthe solid-state laser beam generated by the transition from ⁵D₄ to ⁷F₅and said solid-state laser crystal is doped with no rare-earth ion otherthan Tb³⁺.
 2. A laser-diode-excited solid-state laser apparatusaccording to claim 1, wherein said solid-state laser beam is in awavelength range of 530 to 550 nm.
 3. A laser-diode-excited solid-statelaser apparatus according to claim 1, wherein said GaN-based compoundlaser diode has an active layer made of one of InGaN, InGaNAs, and GaNAsmaterials.
 4. A fiber laser apparatus comprising: a GaN-based compoundlaser diode which emits a first laser beam; and an optical fiber whichhas a core doped with Tb³⁺, and emits a second laser beam generated by atransition from ⁵D₄ to ⁷F₅ when the optical fiber is excited with saidfirst laser beams, wherein the optical fiber includes a first coatingdisposed on a light entrance end surface of the optical fiber and asecond coating disposed on a light exit end surface of the opticalfiber, which enable the optical fiber to emit the second laser beamgenerated by the transition from ⁵D₄ to ⁷F₅ and said solid-state lasercrystal is doped with no rare-earth ion other than Tb³⁺.
 5. A fiberlaser apparatus according to claim 4, wherein said second laser beam isin a wavelength range of 530 to 550 nm.
 6. A fiber laser apparatusaccording to claim 4, wherein said GaN-based compound laser diode has anactive layer made of one of InGaN, InGaNAs, and GaNAs materials.
 7. Afiber laser amplifier comprising: a GaN-based compound laser diode whichemits an excitation laser beam; and an optical fiber which has a coredoped with Tb³⁺, and amplifies incident light which has a wavelengthwithin a wavelength range of fluorescence generated by a transition from⁵D₄ to ⁷F₅ when the optical fiber is excited with said excitation laserbeam, wherein the optical fiber includes a first coating disposed on alight entrance end surface of the optical fiber and a second coatingdisposed on a light exit end surface of the optical fiber, which enablethe optical fiber to amplify the incident light having the wavelengthwithin the wavelength range of fluorescence generated by the transitionfrom 5D0 to 7F2; and said solid-state laser crystal is doped with norare-earth ion other than Tb³⁺.
 8. A fiber laser amplifier according toclaim 7, wherein said fluorescence is in a wavelength range of 530 to550 nm.
 9. A fiber laser amplifier according to claim 7, wherein saidGaN-based compound laser diode has an active layer made of one of InGaN,InGaNAs, and GaNAs materials.